This invention relates to fiber bed elements and processes for removing and collecting small particles of liquids or soluble solids from a gas stream.
It is known to utilize fibers to make fiber bed elements for removing mists containing small particles of liquids or soluble solids from gas streams. The fibers are packed either randomly or in alignment and the gas to be treated is passed through the fiber bed with the small particles of liquids or soluble solids being captured by the fibers in the bed and the gas to be treated has passed through the fiber bed with the droplets being captured by the fibers in the bed. The captured droplets can coalesce on the fibers and form larger drops. The moving gas urges the coalesced drops toward the downstream face of the fiber bed where the liquid drains downwardly under the influence of gravity. Generally, reasonably high collection efficiencies can be achieved with a fiber bed element of this type.
Some of the more frequent applications include removal of acid mists, such as sulfuric acid mist, in acid manufacturing processes; plasticizer mists in, for example, polyvinyl chloride floor or wall covering manufacture; and water soluble solid aerosol such as, for example, emissions from ammonium nitrate prill towers. In removal of water soluble solid aerosols, the collected particulates are dissolved in a liquid within the fiber bed through use of an irrigated fiber bed or of a fogging spray of liquid such as water injected into the gas stream prior to its reaching the fiber bed.
A fiber bed element is designed to collect fine mist particles and then drain them through the structure of the bed. In the design of a mist fiber bed element, the total flow to be processed is specified and the fiber bed element is then designed to provide the required collection efficiency for the mist and to have a pressure drop within the range of acceptable economic performance. The fiber size and thickness of the fiber bed are also chosen in the design procedure. The size of an individual filter or the number of filters is also chosen in the design criteria to meet the overall economic optimum system.
These design practices are well known in the art and are described in detail in various text and patent references. See, for example, the discussion set forth in U.S. Pat. No. 4,086,070.
When the fiber bed element is originally constructed in the factory, it has a dry condition and its characteristics can be measured in the dry condition before it becomes saturated with collected mists. The performance or operation of the element, when it is in the new dry state, is referred to as the dry bed performance or, more particularly, the dry bed pressure drop and collection efficiency at design or operating conditions.
When the element is put into operation, liquid collects in the bed element and is held therein to a certain extent. This liquid hold up results in an additional pressure resistance to gas flow.
In currently used elements, the liquid hold up builds up to a significant level. At steady state operating conditions, mist is draining at the same rate as mist is collecting, and the pressure drop of the saturated or wet fiber bed element will be between 1.6 and 4 times the pressure drop or pressure resistance of the dry bed element as originally constructed. The range of increase, 1.6 to 4.0, can be related to the amount of liquid in the gas stream being processed. The amount of liquid is measured in milligrams of mist material per cubic meter of gas processed. The fiber size and fiber bed voidage also affect the amount of pressure resistance increase.
The increase in pressure resistance is not the result of the insoluble or dirt particles that can build up and add additional pressure resistance. The element, at operating conditions, will operate in this steady state mode with the collected mist or soluble solid mist collecting and simultaneously draining through the fiber bed. At steady conditions, the pressure resistance will be constant over a long period of time.
Another feature generally accepted in currently used fiber bed elements is that liquid flows to the exterior face of the element and flows down on that exterior face until it reaches the bottom of the chamber containing the element. Because the collected liquid drains to the downstream face, reentrainment of this collected liquid can occur. This carryover reentrainment in the cleaned gas results in a lower overall mist control performance.
In filters currently in use such as those described in U.S. Pat. Nos. 4,086,070 and 4,249,918, which disclose roving wound or hand packed fibers, the collected liquid is described as draining through to the downstream face of the collecting fiber and flowing as discreet streams or droplets from the downstream face. Tests performed on filter bed elements such as shown in these patents reflect a pressure drop, when wet with collecting liquid, of approximately 1.6 to 4.0 times the pressure drop existing when the mist filter is dry. This range 1.6 to 4.0 is for liquid mist rates of 0.17 grams of liquid per cubic meter of gas treated up to 10.5 grams of liquid per cubic meter of gas treated. Test data on this type of filter is set forth in the following Table I.
TABLE I ______________________________________ TESTS ON STANDARD WOUND MIST FILTER FILTER SIZE CAGE HEIGHT (meters): 2.440 DIAMETER (meters): 0.610 FIBER BED BED THICKNESS (meters): 0.057 BED DENSITY (Kg/M.sup.3): 173.340 FIBER SIZE (micron): 8.000 BED MIST PRESSURE VELOCITY RATE DROP (KPa) RATIO (M/sec) (g/M.sup.3) DRY BED WET BED (W/D) ______________________________________ 0.197 2.507 1.569 2.764 1.762 0.074 2.684 0.590 1.046 1.772 0.144 2.966 1.145 2.067 1.804 0.197 4.061 1.569 2.789 1.778 0.227 6.462 1.531 2.876 1.878 ______________________________________
There are various techniques used with the elements described in U.S. Pat. No. 4,086,070 to prevent reentrainment of the collected liquid from the exit face of the bed. Sometimes a second layer of fiber is added so that the liquid drains down within the second layer or within large diameter fibers having greater spacing between the fibers.
There are a number of two stage filter beds described in the patent and general technical literature where a second drainage layer is added to the collecting layer to prevent reentrainment of the collected liquid mist. Note the use of a second layer in U.S. Pat. No. 4,086,070.
In another type of mist filter element, two fiber beds are needle punched together. Russian Patent No. 291457 describes the use of a needle punched mist filter bed using two sizes of fiber in the bed, the larger fiber placed on the downstream face thus permitting the collecting liquid to drain from the downstream or exit face of the bed. The needle punching technique is designed to promote liquid flow to the downstream side or downstream face. The actual method of fabricating the element is not described, but the feature of having liquid draining to the downstream face is described. This patent emphasizes the use of a second layer of fiber to prevent reentrainment of the collected liquid.